How to debug a hang while booting the kernel?

Hi,
I'm having trouble to boot a i386 kernel which used to work doesn't
boot into single user mode any more and not into multiuser either.
It hangs right after the wd disk are probed. I don't see the cdrom
which would be one of the next devices to be found and the ehci
attachment is also missing which should also come.
I disabled ehci and other usb deviced via 'boot -c' and removed
the cdrom drive, but that didn't help.
One of the next things to do is running a memory test, but the system
is ussing ECC ram so I expect it to cause some other failures.
Is there a way to find out where the kernel hangs from ddb?
And what do I need to do to find the reason?
I did disable acpi, but then it would fail later during multi
user since i have no isa devices in the kernel and so the isa
devices which has the be there failed. I think it was something
about npx.
Bernd